Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at ambient temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with water and/or molecular oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen, known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen known as oxygenates may be formed during the Fischer-Tropsch process. Hydrocarbons comprising hydrogen and carbon atoms with no unsaturated carbon-carbon bonds are know as paraffins. Paraffins with a straight carbon chain are known as linear paraffins, which include normal alkanes. Paraffins with a branched carbon chain are known as isoparaffins. Isoparaffins comprise isomers of linear paraffins. Isomers are molecules having the same molecular formula as another molecule, but having a different structure and, therefore, different properties. As the carbon atoms in a paraffin molecule increase, the number of possible combinations, or isomers, increases sharply. For example, octane, an 8-carbon-atom molecule, has 18 isomers; decane, a 10-carbon-atom molecule, has 75 isomers. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. In the Fischer-Tropsch process, synthesis gas is catalytically transformed into a hydrocarbon product. The hydrocarbon product primarily comprises normal paraffins. It is generally free of heteroatomic impurities such as sulfur, nitrogen or metals. The hydrocarbon product contains practically no aromatics, naphthenes or, more generally, cyclic compounds, in particular when cobalt catalysts are used. In contrast, the Fischer-Tropsch hydrocarbon product can include a non-negligible quantity of oxygen-containing compounds which, expressed as the weight of oxygen, is generally less than about 10% by weight, and also a quantity of unsaturated compounds (generally olefins) that is generally less than 15% by weight. However, the Fischer-Tropsch product fractions, primarily comprising normal paraffins, cannot be used as they are, in particular because their cold properties are not compatible with the normal use of petroleum cuts. As an example, the pour point of a linear hydrocarbon containing 20 carbon atoms per molecule (boiling point of about 340° C., i.e., usually included in the middle distillate cut) is about +37° C. rendering it impossible to use, as the specification for diesel fuel pour point is −15° C. Fischer-Tropsch hydrocarbon product, mainly comprising linear paraffins, must be transformed into products with a higher added value such as diesel, or kerosene, which are obtained after further hydroprocessing. For example, hydrocarbon waxes from Fischer-Tropsch may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons and/or for conversion to more branched hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons) as well as to enhance some of the cold flow properties of some liquid fuel obtained therefrom.
These processes are well known, but are continually under development in an attempt to enhance the quality of the liquid hydrocarbon products and increase product yields. The embodiments disclosed herein are directed towards these and other related goals.